The modified nucleotide queuosine (Q) is almost universally found in the anticodon wobble position of specific tRNAs. In many bacteria, biosynthesis of Q is modulated by a class of regulatory mRNA elements called riboswitches. The preQ1riboswitch, found in the 5′UTR of bacterial genes involved in synthesis of the Q precursors preQ0and preQ1, contains the smallest known aptamer domain. We report the solution structure of the preQ1riboswitch aptamer domain from Bacillus subtilis bound to preQ1, which is a unique compact pseudoknot with three loops and two stems that encapsulates preQ1at the junction between the two stems. The pseudoknot only forms in the presence of preQ1, and the 3′ A-rich tail of the aptamer domain is an integral part of the pseudoknot. In the absence of preQ1, the A-rich tail forms part of the antiterminator. These structural studies provide insight into riboswitch transcriptional control of preQ1biosynthesis. © 2009 Elsevier Inc. All rights reserved.
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